I have around 800 indexes every 15 days we rebuild
them, how do i know the particular indexes to rebuild
daily as required.

Regular index rebuild are very important. They ensure DBAs have something to do.

However, in general rebuilding indexes regularily to gain performance is the wrong thing to do. It is better to read Tom Kyte's books (see http://oracle.apress.com) or links provided by others to understand why index rebuilds are usually wasted effort. Much better would be to study WHY the performance drops after while.

Exactly. Did anyone gain performance by rebuilding indexes daily? Contrary the performance goes down. This issue has been discussed million times before. Did you go through links from Asktom Citrus gave you?

There is a bunch of articles which explain that rebuild index has a high cost for a performance result not always expected. Two of them here below, to demystify this RoT :
http://www.dbazine.com/oracle/or-articles/jlewis14
http://www.miracleas.dk/images/upload/Docs/Richard%20Foote.pdf

I am giving suggestion to rebuild daily.

Why ? Have you some clues about that ?

With heavy DML's you have to rebuild indexes

I worked on many huge databases with a high rate of insert/delete without have to rebuild indexes to improve notably performance. There are others ways.

If performance suffers then you have to put your experience rather than DOC's

What's the experience if you fall in some Rule of Thumb, which doesn't help at all to solve perf issue.

Given your logic... rebuilding index daily is good, so rebuilding indexes hourly MUST be better... I think the database manages indexes just fine. There are cases when you should rebuild indexes, and a capable DBA should be able to identify them.

For a person to to give the advice to "rebuild your indexes daily" is just ludicrous. Every table and index in a database is unique and has its own characteristics. Why would you rebuild indexes on a table that has static data and hasn't been inserted or updated for months?

I know many consultants who like this attitude. Make the DBA waste all his time so consultants MUST be hired to get anything real accomplished. <g>

With heavy DML's you have to rebuild indexes

Absolutely correct, once in a while - depending on the pattern of DML. For many, many kinds of DML and for most applications, rebuilding indexes is simply a way to prove to the users that "system's unavailable - we are working on it"